Electrostatic photography

ABSTRACT

There is disclosed herein an electrostatic photography method wherein photoconductive sensitive layers are provided on both sides of a support of a photosensitive plate and the reverse side of said plate is illuminated by light either during or before or after corona discharge, to form reproductions on both sides of the photosensitive plate.

United States Patent 151 3,697,172 Ohta et al. [451 Oct. 10, 1972 [54] ELECTROSTATIC PHOTOGRAPHY 2,962,375 11/1960 Schafiert ..96/l.8 X [72] Inventors: wasaburo Ohm Yokohama shi; 2,963,365 12/1960 Grelg ..355/ 17 X Setsuo Soga Tokyo, both of Japan 3,285,740 11/1966 welgl et al ..96/ l 3,443,936 5/1969 Menold .;...250/49.5 X [73] Assgnee- $21 Kmha 3,517,995 6/1970 Gallo et a]. ..2so/49.5 x 221 Filed: July 24, 1969 Primary Examiner-John M. H6166 211 pp No: 44 Assistant Examiner-Alan A. Mathews Attorney-Ernest G. Montague [30] Foreign Application Priority Data [57] ABSTRACT Sept. 9, 1968 Japan ..44/64493 There is disclosed herein an electrostatic photography 7 method wherein photoconductive sensitive layers are [52] US. Cl. ..355/17, 355gb/9lf9/15CC, provided on both sides of a support of a photosensitive 51 1111.01. ..G03g 15/00 Plate and the reverse Side Said Plate is illuminated 581 Field 61 Search ..355/3, 16, 17, 26; 96/1, 1.5, by ligh either during or before or after corona 9 1 g; 7 7 75; 250 495 317 2 2 A discharge, to form reproductions on both sides of the 9 9 9999 991.9991 .1 [56] References Cited UNITED STATES PATENTS 3 3,561,958 2/1971 Kazan .....96/l.5

\\&\ x 1 PATENTEB 1972 3,697,172

INVENTORS 4 14 @911 WWW- ATTORNEY ELECTROSTATIC PHOTOGRAPHY The invention relates to an electrostatic photography method in which a photosensitive plate with-photoconductive layers on both sides of the supporting material is used.

Hitherto, in a photosensitive plate for electrostatic photography with photoconductive layers on both sides of its supporting material, the reverse side or the back side of the photosensitive plate is also charged up in the course of charging the right side of the photosensitive plate by corona discharge or the like, since the photoconductive layer in the reverse side has a high electric resistivity in the dark, even though a material of low electric resistivity is used for the supporting material. Therefore, even after the photoconductive layer on the right side has been exposed to an image and an electrostatic latent image has been formed there, the electric charge on the photoconductive layer of the reverse side does not decay, and there are defects that the non-imaged part of the photosensitive paper becomes heavily smeared and the sharpness and contrast of the image are lost, when the latent image on the photosensitive paper is developed and visualized.

One method to overcome the above-mentioned defects has already been described in Japanese Patent Publication No. 3413/1960. The. photosensitive plate in accordance with this invention is characterized in that photoconductive layers are applied on both sides of its supporting material so that one end portion of each of both sides of the conductive supporting material is exposed.

However, in this invention, the total area of the photosensitive plate cannot be used efficiently, since the conductive material is exposed at one end of the photosensitive plate. Moreover, it is necessary to provide a special electrode in the machine to contact with the exposed conductive supporting material in the case of charging by corona discharge, and in a practical copying machine in which all the processes of charging, exposure to light and development of electrostatic photography should be done within one machine, a complicated equipment is required to bring said electrode in contact with or separate it from said electrically conductive supporting material, whenever the photosensitive plate is put in or out of the machine and also it is moved in the machine.

The present invention is based on a principle completely different from that of the invention referred above, and its object is to use all the surfaces of the photosensitive plate effectively and to provide a method of electrostatic photography in which the nonimaged part of photosensitive paper is not smeared in the course of development.

The principle and characteristic features of this invention are further explained in detail as follows:

Usually, a mixed layer of zinc oxide and a resin or an evaporated layer of selenium is used as a photoconductive sensitive paper for usual electrostatic photography and these photoconductive materials have a relatively high electric resistivity, say, up to more than .Q-cm in the dark while the electric resistivity drops down to 10 Q-cm or less when they are brought into the light, and they show a good conductivity for the discharge current in a corona discharge or the like. The principle of this invention consists in utilizing such characteristics of photoconductive materials, and when an electric charge is applied on a photosensitive plate with photoconductive sensitive layer on both sides of its supporting material, the photosensitive layer on which the charge should not be applied, that is, the photosensitive layer on the reverse side is illuminated by light simultaneously in order to decrease the electric resistivity so that the layer becomes conductive to the discharge current and the barrier to the corona discharge is dissolved. In this way, an efficient and proper charging process is conducted on the photoconductive sensitive layer on the side on which electrostatic latent images should be formed, and at the same time, it suppresses the charging on the reverse side of the photoconductive layer to an extremely low level.

With these and other objects in view, which will become apparent in the following detailed description, the present invention will be clearly understood in connection with the accompanying drawing, in which: FIG. 1 is an enlarged cross sectional view of a photosensitive plate which is used in the present invention;

FIG. 2 is an enlarged cross sectional view of an embodiment in accordance with the invention; and

FIG. 3 is an enlarged cross sectional view of another embodiment of the invention.

FIG. 1 is an embodiment of a photosensitive plate for the electrostatic photography which is used in the invention, and this photosensitive plate 1 is composed of a supporting plate 2 of a paper, a plastic, a metal, or the like with photoconductive sensitive layers 3 and 4 on both sides of the supporting plate. As the photoconductive sensitive layers 3 and 4, those inorganic or organic photoconductive substances similarto those used in the ordinary electrostatic photography, such as selenium, zinc oxide, zinc sulphide, cadmium sulphide, etc. or N- vinyl carbazol, or mixtures of these substances with silicon resin, alkyd resin, vinyl resin, etc., may be used.

FIG. 2 shows an embodiment of a charging device used in the invention, comprising a corona discharge part 5, a high voltage power supply 6, a light source 7 for illumination and an electrode 8 for corona discharge which is almost transparent to light. As this transparent electrode, the so-called NESA glass (trade name) can be used, which is a transparent glass with a NESA coating on it, or a transparent electrode of transparent glass or plastics coated with a thin layer of copper iodide or cadmium oxide on its surface by vacuum evaporation can also be used. Herein, the thin layer 8a is grounded.

In FIG. 2, the above-mentioned photosensitive plate I placed on the transparent electrode 8 undergoes a corona discharge from the corona discharge part 5, and at the same time, a photosensitive layer 4 on the reverse side of the photosensitive plate 1 is exposed to light from below through the transparent electrode 8 by a light source for illumination. In this way, the electric resistivity of the photosensitive layer 4 which is in contact with the transparent electrode drops down, while the photoconductive sensitive layer 3 on the right side is charged up by the corona discharge.

It is needless to say that in this case, by inserting a proper optical system between the light source 7 and the transparent electrode 8, the exposure light can be focused at a part where a corona discharge occurs.

FIG. 3 shows another embodiment of the charging device which is usable in the invention. Corona "discharge parts 9 and 10 are provided according to the power supply 1 l, a light source 12 for illumination, and

an electric shield 13 for the corona discharge parts. In this case, the shield 13 for the corona discharge part 10 has an opening for the exposure light from the light source 12 to pass through it. The shield 13 can be a transparent electrode such as of NESA glass mentioned above. The object of this invention :can also be achieved by the charging device in accordance with the double corona system.

In the above, the method of projecting light from below on the reverse side in the course of corona discharge is explained, but in general, since the reduced electric resistivity of photoconductive material does not recover immediately after the projection of light is turned off, the corona charging process can be conducted after the projection of light on the reverse side of photosensitive plate, or the corona discharge can be done first and the charge on the photosensitive layer on the reverse side can be dissipated through the projection of light only on the reverse side after the corona discharge, making the photosensitive layer conductive, so that 'the charge on the photosensitive layer on the reverse side is substantially diminished.

In summary, according to the invention, a copying on both sides of a photosensitive plate can be done efficiently and an excellent quality image can be obtained on both sides of the photosensitive plate with photoconductive sensitive layers on both sides of its supporting plate in the electrostatic photography.

While the present invention has been particularly described in terms of specific embodiments thereof, it

will be understood that in view of the foregoing specification numerous deviations therefrom and modifications thereupon may be readily devised by those skilled in the art without yet departing from the present teaching.

What is claimed is: l. A method of electrostatic photography comprising the steps of selecting a photosensitive plate having photoconductive sensitive layers on both sides of a supporting plate, uniformly illuminating one side of said photosensitive plate by overall non-image light exposure in order to decrease the electrical resistivity of said one side, charging the other side of said photosensitive plate by a corona discharge, and said illuminating step being conducted during said charging step. 7 2. A method of electrostatic photography comprising the steps of selecting a photosensitive plate having photoconductive sensitive layers on both sides of a supporting plate, uniformly illuminating one side of said photosensitive plate by overall non-image light exposure in order to decrease the electrical resistivity of said one side, charging the other side of said photosensitive plate by a corona discharge, and said illuminating step being conducted before said charging step. 

2. A method of electrostatic photography comprising the steps of selecting a photosensitive plate having photoconductive sensitive layers on both sides of a supporting plate, uniformly illuminating one side of said photosensitive plate by overall non-image light exposure in order to decrease the electrical resistivity of said one side, charging the other side of said photosensitive plate by a corona discharge, and said illuminating step being conducted before said charging step. 